Coatings applied from aqueous medium have become of increasingly greater importance, but the aqueous coatings known to the art have significant limitations, as will be discussed.
Aqueous emulsion copolymers have been used in latex form, but these systems are suspensions of polymer particles which are stabilized in the aqueous medium by surfactants, and the surfactants are water sensitive and limit the utility of the coatings. Also, most latex coatings are thermoplastic, and this limits the solvent resistance and physical toughness of the coatings. Moreover, efforts to directly use the colloidal dispersions containing anionic or cationic surfactants in electrocoating have failed because film build is excessive and the deposited films are porous.
It has also been possible to disperse relatively low molecular weight copolymers made by polymerization in organic solvent solution in water by including carboxylic acid groups or amine groups in the relatively low molecular weight copolymer which is formed. Such copolymers containing the described groups which will form salts when reacted with acids or bases were then dispersed in water in salt form with the aid of a water-miscible organic solvent. The resulting aqueous compositions might be characterized as solutions or dispersions, depending upon the particle size and clarity thereof. In normal practice, a reactive monomer, such as an hydroxyl-functional monomer is also included in the copolymer, and these copolymers are useful to provide thermosetting coatings after the inclusion of a curing agent in the aqueous medium.
Unfortunately, the desired properties in these coatings had to be provided by the cure, for the copolymers which were dispersed in water by salt formation in the presence of organic solvent had to be of low molecular weight and an extensive cure was needed to build the molecular weight and cross-link density needed to provide a physically tough film. This required excessive amounts of curing agents, thus limiting the balance of hardness, flexibility and chemical resistance which could be obtained.
It is desired to use partially cross-linked copolymers of higher molecular weight which are substantially free of surfactants so that the use of curing agents can be either minimized or eliminated. In this way, copolymers having new properties are provided which form superior coatings and which can be added to electrocoat baths to improve coating performance, especially salt spray resistance and edge corrosion protection.